How can I close the gap in the mouth?
While in edit mode, press  Ctrl  +  Tab  and select Edge. Then press  Alt +RMB to select the edge loop in question. Finally, press  F  to create a new face.

Alternatively, you may be able to select the upper and lower edges, bring them closer together until they're nearly touching and press  W  then remove doubles, and adjust your merge distance in the operator panel until you are satisfied with the results.

You may also want to use proportional editing for this kind of task. Press O to enable it, then use the scroll wheel to adjust the strength during a transform operation.